1. Field of the Invention
The present invention relates to a mold cap for a semiconductor device mold package, and more specifically to a mold cap for a semiconductor device mold package composed of the mold cap and a mold case both of which are formed of a resin compound and which are joined to each other to form an inner hollow space defined and encapsulated by the mold cap and the mold case.
2. Description of Related Art
Referring to FIG. 1, there is shown a diagrammatic sectional view of a mold cap for a semiconductor device mold package, which is disclosed by Japanese Patent Application Pre-examination Publication No. JP-A-58-164230, an English abstract of which is published from the Japanese Patent Office. The content of the English abstract of JP-A-58-164230 is incorporated by reference in its entirety into this application.
In the shown mold cap generally designated with Reference Numeral 3, since a design of an outer wall height xe2x80x9caxe2x80x9d and a mold cap resin thickness xe2x80x9ctxe2x80x9d have not been considered when the mold cap is designed, various problems have been encountered because of a thermal stress generated in the mold cap 3 in the process for assembling and bonding the mold cap and a mold case (not shown) to accommodate a semiconductor chip (not shown) in an inner space defined and encapsulated between the mold cap and the mold case. A first problem is that, because of the thermal stress, the mold cap 3 notably deforms in shape, so that a crack often occurs in the mold cap 3 or a bond strength between the mold cap and the mold case significantly drops. A second problem is that, when an ambient temperature elevates, a gas encapsulated in the inner hollow space of the semiconductor device mold package expands, so that the mold cap 3 is often blown away.
The cause for the above mentioned problems are considered as follows:
A cause for the first problem is considered as follows: Since there is a considerable difference between an outer wall surface length xe2x80x9cLoutxe2x80x9d (=2a+W) and an inner wall surface length xe2x80x9cLinxe2x80x9d, a difference occurs in a linear expansion length between the outer wall surface length xe2x80x9cLoutxe2x80x9d and the inner wall surface length xe2x80x9cLinxe2x80x9d due to temperature variation. Specifically, when the ambient temperature drops, the mold cap 3 deforms to open a mouth of the mold cap. To the contrary, when the ambient temperature elevates, the mold cap 3 deforms to close or purse the mouth of the mold cap.
A cause for the second problem is considered as follows: When the ambient temperature elevates, the pressure of the gas encapsulated in the inner hollow space of the semiconductor device mold package, increased by the ambient temperature elevation, becomes superior to the bond strength of the mold cap.
Accordingly, it is an object of the present invention to provide a mold cap for a semiconductor device mold package, which has overcome the above mentioned defects of the conventional one.
Another object of the present invention is to provide a mold cap for a semiconductor device mold package, and configured to minimize the deform of the mold cap caused by the thermal stress, thereby to prevent generation of a crack in the mold cap.
Still another object of the present invention is to provide a mold cap for a semiconductor device mold package, and configured to minimize the expansion of the gas encapsulated in the package inner hollow space, caused by the ambient temperature elevation, thereby to prevent the mold cap from being blown away.
The above and other objects of the present invention are achieved in accordance with the present invention by a mold cap for a semiconductor device mold package composed of the mold cap and a mold case both of which are formed of a resin compound and which are joined to each other to form a hollow inner space defined and encapsulated by the mold cap and the mold case, the mold cap having a recess defined by an inner wall surface and surrounded by a mouth portion, a height from the mouth portion to a top of an outer wall surface being defined as xe2x80x9caxe2x80x9d, a height from the mouth portion to a top of the inner wall surface being defined as xe2x80x9cbxe2x80x9d, and a thickness of the mold cap at the mouth portion being defined as xe2x80x9ctxe2x80x9d, and the mold cap being constructed to fulfill a relation of a-bxe2x89xa7t and a-bxe2x89xa6b.
In an embodiment of the mold cap, the inner wall surface of the mold cap is shaped to draw a quadrilaterals, for example, a rectangle, in a vertical section passing through a center of the mold cap, and the outer wall surface of the mold cap is shaped also to draw a quadrilaterals, for example, a rectangle, in the vertical section passing through the center of the mold cap. Specifically, in a bottom view, the mouth portion has an outer contour in the form of a circle, and an inner contour in the form of a circle, so that the mold cap is in a cylindrical form, and the inner wall surface confines a recess in a cylindrical form.
Alternatively, the inner wall surface of the mold cap is shaped to draw a smoothed curve line, such as a portion of a circle or an ellipse, in a vertical section passing through a center of the mold cap, and the outer wall surface of the mold cap is shaped to draw a quadrilaterals such as a rectangle in the vertical section passing through the center of the mold cap. Specifically, in a bottom view, the mouth portion has an outer contour in the form of a circle, and an inner contour in the form of a circle, so that the mold cap is in a cylindrical form, and the inner wall surface confines a recess in a partial spherical body form, for example as one obtained by rotating a partial circle or ellipse defined by an arc and a chord joining the ends of the arc, around an axis passing though a center of the chord perpendicularly to the chord. Preferably, the outer wall surface has a chamfered corner in the vertical section passing through the center of the mold cap.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.